In the manufacture and fabrication of gas panels many different types of materials are used, and in numerous types of such construction very small and very thin parallel electrical conductors are deposited on substrates, and a pair of such substrates are disposed on opposite sides of a chamber filled with an illuminable gas with one set of the parallel electrical conductors extending orthogonally with respect to the other set of parallel electrical conductors to define coordinate intersections. A given one of the parallel electrical conductors from each set is energized with electrical signals to ignite the region of the illuminable gas around a selected coordinate intersection. The electrical conductors are made as narrow and thin as desired in order to obtain a greater number of coordinate intersections per square inch of the gas panel.
Often times however, commercial gas panels entail, in part, the use of a conductor array fabricated by applying to a glass plate, a chrome layer, followed by a copper layer, followed by another chrome layer. Also, sometimes a cermet layer is placed between the glass plate substrate and bottom chrome layer. Next, a photoresist composition is applied so that selected areas of the chrome/copper/chrome/cermet layers can be removed to provide the desired electrical connections on the substrates.
Glass is deposited over the top chrome layer to ensure separation of the parallel lines. The copper layer provides electrical conductivity. The bottom chrome layer is applied to ensure adequate adhesion between the copper and the glass or cermet. The cermet in conjunction with subsequently applied material acts as a resistor in the final product.
The etching of the chrome layers has been carried out employing etchant compositions having a high pH, such as aqueous compositions containing KMnO.sub.4. The use of aqueous etchant compositions having a high pH is not entirely satisfactory, since KMnO.sub.4 tends to attack the photoresist to some extent as well as the chrome layers.
Further, commercially available positive photoresist materials such as those based on phenolic-formaldehyde novalak polymers are not resistant to the highly basic etchant compositions employed to etch the chrome, and, accordingly will not protect the non-etched areas.
Therefore, the ability to employ a positive photoresist is advantageous for the reason that a positive resist is less sensitive to dirt or other contaminants than is a negative photoresist, because only the exposed areas of a positive photoresist are developed and etched away. Consequently, if dirt or another contaminant matter is present, it will remain on the unexposed portion, and will not play a significant part in regard to formation of defects. By contrast, with a negative photoresist, the exposed areas are cured and the unexposed areas are etched away.
In addition, the ability to use a positive photoresist makes it possible to employ a single coating to prepare several different circuits by exposing, developing and etching the required surface and then repeating the steps as many times as needed. Furthermore, positive resists provide sharper image resolution as compared to negative resists, since the desired image does not swell and remains unchanged during the development with the particular solvent. Further still, the unexposed positive photoresist can be readily removed when desired by simple chemical solvents which include N-methyl-2-pyrrolidone for many commercially available positive resists and/or reexposed to suitable light, and then removed with the same solution employed to develop the circuitry. Additionally, aqueous solutions that are highly basic such as sodium hydroxide can also effectively remove the photoresist.
However, the various commercially available positive photoresists, such as the methacrylate polymers, necessitate an etchant for the underlying chrome which is on the acidic side. Although certain acidic etchants have been suggested for chrome, they are not entirely satisfactory. For instance, the etching with various prior acidic etchants is very slow at the start but then accelerates very rapidly forming or generating relatively large amounts of gas which are uncontrollable and cause the formation of bubbles. This is not suitable, especially for fine line circuitry. Moreover, sometimes the chrome surface is not even etched at all in such acidic etchants which may be possible due to passivation of the chrome surface.
It has previously been found that mixtures of glycols and diluted HCl etch chromium at ambient temperatures. But, when etching the lower chromium layer, the exposed edges of the top chromium layer etch during the relatively longer times needed to etch the lower layer so that undercutting of the top layer occurs. This in turn results in a portion of the copper being exposed at the top edges and ends of the conductor lines, and this could render the structure inoperative upon subsequent or further fabrication.
It has also been found that a concentrated HCl mixture of about 50% or more by volume provides an etch time for the lower chromium layer which sufficiently minimizes the undercutting of the top chromium layer; however, these concentrated mixtures attack the cermet, when used, and cause unacceptable changes in its resistivity.
These problems of undercutting of the top chromium layer and changes in the resistivity of the cermet have been minimized by employing the invention disclosed in U.S. Pat. No. 4,160,691 to Abolafia, et al. wherein certain concentrated HCl compositions are employed at temperatures from about 50.degree. to about 95.degree. C. Although, the invention disclosed and described in U.S. Pat. No. 4,160,691 does minimize undercutting of the top chromium layer and the changes in resistivity of the cermet occasioned by other acidic compositions, it is still not entirely satisfactory. The compositions are still in need of improvement with respect to pH stability during use, and improvement with respect to storage stability over relatively long periods of time would still be useful.
Most importantly, however, is the fact that etching the top and bottom chrome layers of a chrome-copper-chrome conductor array deposited on a glass plate employing prior art etchants such as potassium permanganate entailed the numerous steps of: contacting the upper chrome layer with potassium permanganate and washing off excess permanganate with water; contacting the intermediate copper layer with ammonium persulfate and washing off the persulfate with water; and contacting the lower chrome layer with potassium permanganate. However, in the case of the lower chrome layer, it is necessary to remove the manganese component of the potassium permanganate etchant even after the rinse with water, by using an oxalic acid rinse followed by a water rinse. Thereafter, a second etching step using more potassium permanganate was required, and this was followed by a final water rinse.
It is believed that the lower chrome layer next to the glass or cermet has a higher oxide content than the upper chrome layer and is more difficult to etch. Therefore to ensure thorough etching, it was necessary to remove the manganese component of the potassium permanganate from the lower chrome layer with oxalic acid, and conduct another or second etching step on the lower chrome layer with additional potassium permanganate.